Butt problems on my character model
I am not sure, but for me it looks like some of the faces are not split where they should be. On the low poly model it looks like when you are going from the lower back to the butt that the first face ( the one that is on the screenshot "over the red line") is not split in two verticaly. And also the face three below that. I cannot clearly see due to the perspective. 

Optionally I could imagine that you maybe have 2 vertecies over each other so try and remove doubles, but I guess you already did that.
Hope that helped :)